Microwave rotary ring joint



March 14, 1961 w, E, QMM ETAL 2,975,382

MICROWAVE ROTARY RING JOINT Filed May 24, 1957 2 Sheets-Sheet 1 INVENTORS w. a FA0/7/7 March 14, w. E FROMM ET AL MICROWAVE ROTARY RING JOINT Filed May 24, 1957 2 Sheets-Sheet 2 3 an F (/wr/wva 4- m/ H I 1 4/ (mum a fa United States Patent MICROWAVE ROTARY RING JOINT Winfield E. Fromm Hempstead, and Eugene G. Fubini, Glen Head, N. assignors to the United States of imerica as represented by the Secretary of the Air orca Filed May 24, 1957, Ser. No. 661,553

3 Claims. (Cl. 333-98) This invention relates to rotary joints for high frequency energy and particularly to a rotary joint having a passageway containing its axis of rotation. This characteristic permits a number of such joints to be stacked about a common axis of rotation, with stationary feed lines passing through the axially aligned passageways, which makes possible a plurality of high frequency channels interconnecting stationary equipment and equipment mounted on a rotating pedestal. In many radar applications it is necessary to have a plurality of antennas mounted on a single rotating structure and connected with stationary transmitting and receiving equipment.

Where rotary joints are used in MTI (moving target indication) radar systems, which sense the Doppler phase shift produced by moving targets, it is essential that the phase shift produced by the joint be the same at all angular positions of the coupling. A primary object of the invention is to produce a rotary joint of the above type in which rotation of the joint produces no interference with the phase of the transmitted energy. Other objects of the invention are to provide a rotary joint of the above type in which the amplitude of the transmitted wave is independent of joint rotation, which introduces a minimum of discontinuities or mismatches leading to standing waves in the transmission system and which is capable of handling high power.

Basically the rotary joint consists of a large central coaxial coupling cavity. in which the center conductor is hollow and provides the passageway for feed lines and the relatively rotating sections of which are fed by similar transmission systems each consisting of a primary binary feed coupled to two secondary binary feeds which in turn are coupled to four probes located 90 apart around the central coaxial coupling. The invention will be described in more detail in connection with the specific embodiment thereof shown in the accompanying drawings in which:

Fig. 1 is a plan view partly in section of the rotary joint;

Fig. 2 is an elevation in section of the rotary joint; and

Fig. 3 shows the method of stacking and feeding a plurality of rotary joints.

Referring to Figs. 1 and 2, the central coaxial coupling cavity has an upper section 1 and a lower section 2. The outer conductor is designated 3 and the inner conductor is designated 4 in each section. The upper and lower sections are coupled by a rotary joint, the annular breaks in the outer and inner conductor surfaces at which relative rotation occurs being designated 5 and 6, respectively. These breaks form the inputs of half-wave shorted transmission lines, designated 7 and 8, line 7 being folded, where the impedance and therefore the voltage are theoretically zero. Bearings 9 and 10 are located at the midpoints of the half-wave shorted lines where the current is theoretically zero. Rotary junctions of this type are well known in the art and described in the literature, for example, volume 9 of the Radiation Laboratory Series, Microwave Transmission Circuits, pages 100-114, Mc-

Patented Mar. 14, 1961 Graw-I-Iill, 1948. The hollow interior of inner conductor 1 provides a passageway for feed transmission lines, four such lines, in the form of rectangular waveguides numbered 11, 12, 13 and 14, being shown in the drawing. Waveguide 11 makes a right angle bend just above coaxial coupling section 1 and serves to feed the rotary joint shown as will be explained later. The other guides serve as feeds to joints stacked above the one shown in the manner illustrated in Fig. 3.

Waveguide 11, which forms a right angle bend above coaxial coupling section 1, terminates in a rectangular waveguide T-junction having equal arms 15 and 15'. Arm

I5 terminates in a waveguide to coaxial transition 16, the coaxial line having outer conductor 17 and inner conductor 18. This line terminates in a coaxial T-junction havng equal coaxial arms 19 and 19'. The outer conductors of arms 19 and 19 are equal in diameter to outer conductor 17; the inner conductors, however, are smaller in diameter than inner conductor 18 by the amount required to make the characteristic impedance of each arm twice that of line 17-18. Arm l9 terminates in a matched transition to upper section 1 of the central coaxial coupling cavity, generally indicated at 20. Arm 19' is similarly coupled to upper section 1 at a point removed from the arm 19 coupling. In an identical manner to that just described for arm 15, arm 15' is coupled to upper section 1 of the coupling cavity at two points separated from each other and from the two points fed by arm 15 by 90. It is apparent from this construction that the distance travelled between waveguide 11 and the four coupling points is the same for each point which results in all four points operating at the same phase and amplitude.

Lower section 2 of the central coaxial coupling cavity is coupled to the channel #2 waveguide in exactly the same manner as the coupling between waveguide 11 and upper section 1, except for the greater radius of curvature of waveguides 15" and 15'" and the necessarily greater lengths of coaxial lines 17" and 17'. In Fig. 1 most of this structure is located directly beneath the corresponding structure for the upper section, however it is identical in appearance. As in the upper section, the phases and amplitudes at the four feed points in the lower section are the same.

Fig. 3 shows the method of stacking a plurality of the rotary joints illustrated in Figs. 1 and 2 with a common axis of rotation. The lowest joint in the stack is inverted from the position shown in Figs. 1 and 2 in order to conserve space in the central passageway. It is a matter of choice whether other joints are inverted. The mechanical supporting structure is not shown for simplicity, however it may take many forms. In its simplest form the bundle of feed waveguides passing through the central passageways of the rotary joints may be made sufiiciently rigid in some cases to support the entire structure including the rotating antennas. In this case the upper section 1 of the rotary joint would be supported directly from the waveguide bundle by direct attachment and the antennas would be carried by the rotating lower section 2 of the joint and associated structure. Another arrangement is to have the central passageways surround a hollow mast containing the feed waveguides. In this case the sections 1 would be supported from the mast and the antennas would either be supported from the lower sections 2 or else from structures bearing directly on the mast. If the central passageway is sufficiently large or the number of feed waveguides sufficiently small and not too great in number, the waveguides may be supported on the outer surface of the mast. A third method would be to have a rotating pedestal large enough to completely surround the stack of Fig. 3. The antennas and rotating portions of the joints would in this case be attached to the pedestal.

What is claimed is:

l. A ring type rotary joint for high frequency waves comprising a coupling element in the form of a length of coaxial transmission line having inner and outer cylindrical conductors electrically connected at their ends to form a cavity, said inner conductor being of large diameter, hollow and open at the ends whereby a coaxial cylindrical passageway is provided therethrough, said inner and outer conductors having rotary joints intermediate their ends dividing said coupling element into first and second relatively rotatable sections, a first transmission line, coupling means between said first transmission line and the first section of said coupling element, said coupling means providing equal transmission and equal phase shift between said first transmission line and four Points equally spaced about the circumference of the first section of said coupling element, a second transmission line, and coupling means between said second transmission line and the second section of said coupling element, said last named coupling means providing equal transmission and equal phase shift between said second transmission line and four points equally spaced about the circumference of the second section of said coupling element.

2. A ring type rotary joint for high frequency waves comprising a coupling element in the form of a length of coaxial transmission line having inner and outer cylindrical conductors connected at their ends to form a cavity, said inner conductor being of large diameter, hollow and open at the ends whereby a coaxial cylindrical passageway is provided therethrough, said inner and outer conductors having rotary jointsintermediate their ends dividing said coupling elements into first and second relatively rotatable sections, a first rectangular waveguide terminating in a rectangular waveguide T-junction having equal arms each terminating in a rectangular waveguide to coaxial transmission line junction, said coaxial lines being of equal length and each terminating in a coaxial line to coaxial line T-junction having equal arms each terminating in a coupling to the first section of said coupling element at one of four points equally spaced about its circumference, a second rectangular waveguide terminating in a rectangular waveguide T-junction having equal arms each terminating in a rectangular waveguide to coaxial transmission line junction, said coaxial lines being of equal length and each terminating in a coaxial line to coaxial line T-junction having equal arms each terminating in a coupling to the second section of said coupling element at one of four points equally spaced about its circumference.

3. A multiple channel rotary joint assembly comprising a plurality of individual single channel rotary joints as claimed in claim 1 in which said individual joints are stacked in axial alignment and in which one of said first and second transmission lines associated with each of said stacked individual joints passes through the said cylindrical passageway in each of the individual joints situated between that particular individual joint and one end of said stack.

References Cited in the file of this patent UNITED STATES PATENTS 2,473,443 Ragan June 14, 1949 2,523,348 White et a1. Sept. 26, 1950 

